std::ranges::is_permutation
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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template< std::forward_iterator I1, std::sentinel_for<I1> S1,
std::forward_iterator I2, std::sentinel_for<I2> S2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_equivalence_relation<std::projected<I1, Proj1>,
std::projected<I2, Proj2>>
Pred = ranges::equal_to >
constexpr bool
is_permutation( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );
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(1) | (since C++20) |
template< ranges::forward_range R1, ranges::forward_range R2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_equivalence_relation
<std::projected<ranges::iterator_t<R1>, Proj1>,
std::projected<ranges::iterator_t<R2>, Proj2>>
Pred = ranges::equal_to >
constexpr bool
is_permutation( R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );
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(2) | (since C++20) |
Checks whether the first target range [first1, last1) or r1 is a permutation of the second target range [first2, last2) or r2. Elements of the two ranges are projected by proj1 and proj2 respectively, and equality is determined by pred.
The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:
- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.
Parameters
| first1, last1 | - | the iterator-sentinel pair defining the first target range |
| r1 | - | the first target range |
| first2, last2 | - | the iterator-sentinel pair defining the second target range |
| r2 | - | the second target range |
| pred | - | the predicate to be applied to the (projected) elements |
| proj1 | - | the projection to be applied to the elements in the first target range |
| proj2 | - | the projection to be applied to the elements in the second target range |
Return value
true if the first target range is a permutation of the second target range, false otherwise.
Complexity
Given N as ranges::distance(first1, last1) or ranges::distance(r1):
1,2) 𝓞(N2) applications of
pred, proj1 and proj2 (or only exactly N applications if the two target ranges are lexicographically equal).If I1, S1, I2 and S2 pairwise model sized_sentinel_for (or both R1 and R2 model sized_range), and N does not equal ranges::distance(first2, last2) or ranges::distance(r2), then no comparison will be made.
Notes
std::is_permutation can be used in testing, namely to check the correctness of rearranging algorithms (e.g. sorting, shuffling, partitioning). If x is an original range and y is a permuted range then std::is_permutation(x, y) == true means that y consist of the same elements, maybe staying at other positions.
Possible implementation
struct is_permutation_fn
{
template<std::forward_iterator I1, std::sentinel_for<I1> S1,
std::forward_iterator I2, std::sentinel_for<I2> S2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_equivalence_relation<std::projected<I1, Proj1>,
std::projected<I2, Proj2>>
Pred = ranges::equal_to>
constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2,
Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
// skip common prefix
auto ret = ranges::mismatch(first1, last1, first2, last2,
std::ref(pred), std::ref(proj1), std::ref(proj2));
first1 = ret.in1, first2 = ret.in2;
// iterate over the rest, counting how many times each element
// from [first1, last1) appears in [first2, last2)
for (auto i {first1}; i != last1; ++i)
{
const auto i_proj {std::invoke(proj1, *i)};
auto i_cmp = [&]<typename T>(T&& t)
{
return std::invoke(pred, i_proj, std::forward<T>(t));
};
if (i != ranges::find_if(first1, i, i_cmp, proj1))
continue; // this *i has been checked
if (const auto m {ranges::count_if(first2, last2, i_cmp, proj2)};
m == 0 or m != ranges::count_if(i, last1, i_cmp, proj1))
return false;
}
return true;
}
template<ranges::input_range R>
constexpr auto get_end(R&& r)
{
return ranges::end(r);
}
template<ranges::forward_range R>
constexpr auto get_end(R&& r)
{
return ranges::next(ranges::begin(r), ranges::end(r));
}
template<ranges::forward_range R1, ranges::forward_range R2,
class Proj1 = std::identity, class Proj2 = std::identity,
std::indirect_equivalence_relation
<std::projected<ranges::iterator_t<R1>, Proj1>,
std::projected<ranges::iterator_t<R2>, Proj2>>
Pred = ranges::equal_to>
constexpr bool operator()(R1&& r1, R2&& r2,
Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
return (*this)(ranges::begin(r1), get_end(r1),
ranges::begin(r2), get_end(r2),
std::move(pred), std::move(proj1), std::move(proj2));
}
};
inline constexpr is_permutation_fn is_permutation{};
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Example
Run this code
#include <algorithm>
#include <array>
#include <cmath>
#include <iostream>
#include <ranges>
auto& operator<<(auto& os, const std::ranges::forward_range auto& v)
{
os << "{ ";
for (const auto& e : v)
os << e << ' ';
return os << "}";
}
int main()
{
static constexpr auto r1 = {1, 2, 3, 4, 5};
static constexpr auto r2 = {3, 5, 4, 1, 2};
static constexpr auto r3 = {3, 5, 4, 1, 1};
static_assert
(
std::ranges::is_permutation(r1, r1) &&
std::ranges::is_permutation(r1, r2) &&
std::ranges::is_permutation(r2, r1) &&
std::ranges::is_permutation(r1.begin(), r1.end(), r2.begin(), r2.end())
);
std::cout
<< std::boolalpha
<< "is_permutation(" << r1 << ", " << r2 << "): "
<< std::ranges::is_permutation(r1, r2) << '\n'
<< "is_permutation(" << r1 << ", " << r3 << "): "
<< std::ranges::is_permutation(r1, r3) << '\n'
<< "is_permutation with custom predicate and projections: "
<< std::ranges::is_permutation
(
std::array {-14, -11, -13, -15, -12}, // 1st range
std::array {'F', 'E', 'C', 'B', 'D'}, // 2nd range
[](int x, int y) { return abs(x) == abs(y); }, // predicate
[](int x) { return x + 10; }, // projection for 1st range
[](char y) { return int(y - 'A'); } // projection for 2nd range
)
<< '\n';
}
Output:
is_permutation({ 1 2 3 4 5 }, { 3 5 4 1 2 }): true
is_permutation({ 1 2 3 4 5 }, { 3 5 4 1 1 }): false
is_permutation with custom predicate and projections: true
See also
(C++11) |
determines if a sequence is a permutation of another sequence (function template) |
| generates the next greater lexicographic permutation of a range of elements (function template & algorithm function object) | |
(C++20) |
|
| generates the next smaller lexicographic permutation of a range of elements (function template & algorithm function object) | |
(C++20) |
|
(C++20) |
specifies that a relation imposes an equivalence relation (concept) |